Low-pressure arc discharge lamps, such as fluorescent lamps, are well known in the art and typically include a pair of cathodes made of a coil of tungsten wire upon which is deposited a coating of an electron-emissive material consisting of alkaline metal oxides (i.e., BaO, CaO, SrO) to lower the work function of the cathode and thus improve lamp efficiency. With electron-emissive material disposed on the cathode filament, the cathode fall voltage is typically about 10 to 15 volts. However, at the end of the useful life of the lamp when the electron-emissive material on one of the cathode filaments becomes depleted, the cathode fall voltage quickly increases by 100 volts or more. If the external circuitry fails to limit the power delivered to the lamp, the lamp may continue to operate with additional power being deposited at the lamp cathode region. By way of example, a lamp which normally operates at 0.1 amp would consume 1 to 2 watts at each cathode during normal operation. At end-of-life, the depleted cathode may consume as much as 20 watts due to the increase in cathode fall voltage. This extra power can lead to excessive local heating of the lamp and fixture.
Small diameter (e.g., T2 or 1/4 inch) fluorescent lamps generally have very high ignition voltage requirements necessitating the use of ballasts with open circuit output voltages which may exceed 1000 volts. Such voltage levels are enough to sustain a conducting lamp with an arc drop of 50 to 150 volts with a depleted cathode and an end-of-life cathode fall voltage of 200 volts. In this example, the lamp would run at nearly rated current because the excess voltage would be mostly dropped across the output impedance of the ballast. Since the cathodes in these small diameter T2 lamps are placed much closer to the internal tube wall than in larger diameter lamps, less cathode power is needed to overheat the glass in the area of the cathode. In such T2 diameter lamps, it would be desirable to limit the increase in cathode power to 6 watts in order to avoid excessive local heating.
For a 6 watt increase in cathode power, the corresponding RMS lamp voltage increase is only about 52 volts. Normal lamp voltage varies with lamp length, production variation, cathode heating, ambient temperature, and fixture effects and can easily vary by 50 volts or more. For example, the lamp voltage of a typical 13 watt T2 diameter lamp during normal operation may vary from 115 volts to 165 volts.
Various attempts have been made to provide overvoltage or over-current protection in inverter-type ballasts in order to prevent circuit damage due to excessive load power. For example, U.S. Pat. No. 5,262,699, which issued to Sun et al on Nov. 16, 1993, describes an inverter-type ballast having means for detecting a relatively large increase in current resulting from a resonant mode or open circuit (i.e. no load) condition. The inverter is disabled whenever the lamp is removed or if the lamp fails to ignite. Depletion of emissive material on one or more of the lamp electrodes, which prevents the lamp from igniting, will cause such an open circuit condition.
U.S. Pat. No. 4,503,363, which issued to Nilssen on Mar. 5, 1985, describes an inverter-type ballast having a subassembly which senses the voltage across the output of the ballast. When an open circuit condition is detected at the input of the subassembly, resulting from the removal of a lamp from one of its sockets or the failure of a lamp to ignite, the inverter is disabled.
While the disabling circuits of U.S. Pat. Nos. 5,262,699 and 4,503,363 may be effective at disabling the inverter upon detection of a relatively large increase in current or voltage, these circuits are ineffective at responding to relatively small increases in cathode fall power.
"Quicktronic" inverter ballasts manufactured by OSRAM GmbH for operating "Dulux DE" compact fluorescent lamps monitor an increase in ballast input power by sensing supply voltage which is boosted with RF feedback from the lamp. Effectively, lamp voltage is sensed since lamp current is somewhat constant in the ballast over the sense range i.e., voltage=power/current. An increase in input power of about 6 to 10 watts with a .+-.2 watt tolerance is required to disable the inverter. Due to the drawbacks of voltage sensing as discussed above, this approach is best suited for sensing very large voltage increases such as a lamp no start or open circuit load condition. Moreover, this approach requires tight control of circuit component tolerances which adds to cost and reduces load flexibility. Finally, this approach is not easily adapted to a multiple lamp configuration because it is difficult to sense lamps independently.